I did some checking, it appears that the switch over happened sometime in late April of '42.
here's a picture from Fighting 3 off the Lexington in April of '42
Note the star on the port wing.
Here's some of planes on the Enterprise in April of '42:
These already have stars on both wings.
A shot of Wildcats on the Hornet in April of '42:
It's hard to tell, but they also appear to have stars on the tops of both wings, so we know it wasn't just the Enterprise
Immediately after the battle, Fighting 6 on the Enterprise, May 12, '42
It's hard to see, but again, they appear to have stars on both wings. The change to the star without the Meatball hasn't happened yet.
All of this suggests that, at the time of the Battle of the Coral Sea, the fleet was using the star and ball roundels on both wings. However, I could not find pictures of Lexington or Yorktown aircraft from between the date of that first pic and the battle, so I don't know for sure that they changed when the rest of the fleet changed.